Gauge: As big as you can/want to work with, within limit. Ideally:
- 18, 20 or 22 gauge for signal data (eg buttons, joysticks, LEDs)
- 14, 16 or 18 for DC power/incandencent bulbs
- 12, 14 or 16 for AC power.
If you're going to scrimp do it on buttons/joys not on power. Power pulls lots of amps and needs a thicker wire. Data pulls miliamps and doesn't need much at all, but too thin and you'll end up breaking it off connectors/pins all the time.
I believe Bob Roberts suggest 16 for AC, 18 for DC and 20 for data. I personally would do 14 / 16 / 18 myself.
I would use the AMP crimp-on connectors that are fully shielded with a rubber end for ALL connections. (Bob has these), or at the very least the hard plastic partially shielded ones from Home Depot/Lowes/Menards (GC brand). If you ever want to move stuff or diagnose, you don't want to have to desolder, and if your soldering skills arn't good, crimping is the way to go (though you do need to know how to properly crimp)
For connection into the IPAC (I think that has screw barrier strips right?) I would pre-tin ALL the lead ends (tinning means twisting the end and flowing solder into the bare wire to keep it from unraveling and to provide good all around contact).